<span>D. {0} </span> If you multiply two numbers from a closed set the answer is in the set. <span>So A is not closed because 2 x 2 = 4 which isn't in the set. Only 0.5, 1, and 2 are in set A. </span>
<span>Try multiplying the largest numbers in sets B, C, and D by themselves. You will soon discover the closed set.</span>